Ultra-wideband (UWB) communication has attractive features for baseband multiple access, tactical wireless communications, and multimedia services. In general, an UWB transmission consists of a train of very short pulses occupying an ultra-wide bandwidth. The information is typically encoded via either linear pulse amplitude modulation (PAM) or nonlinear pulse position modulation (PPM). The ultra-wide bandwidth includes bandwidths that are licensed from the Federal Communication Commission (FCC) for other communication purposes. However, the short pulses of the UWB transmission appear as minimal noise to narrowband systems operating within those licensed frequencies.
Conveying information over ultra-short waveforms allows UWB systems to provide low-power low-complexity baseband operation, ample multipath diversity, and a potential to enhance user capacity. These features make UWB connectivity suitable for indoor and especially short-range high-rate wireless links in the workplace and at home. To achieve these features, UWB systems must be able to accommodate multiple users in the presence of narrowband interference (NBI) introduced by the overlaid existing narrowband systems.
UWB systems may rely on spreading schemes to enable multiple access. Existing baseband, i.e., carrier-less, spreading schemes rely on time-hopping (TH) or direct-sequence (DS) codes. These codes can lead to constant-modulus transmissions, but they are not substantially flexible in handling multi-user interference (MUI) and NBI with low-complexity receivers, which are two critical factors limiting performance of UWB systems in the presence of multipath and co-existing narrowband services.